1. Field of the Invention
The present invention is related to a thread joint for connecting tubes which are used for test excavation, production or the like of oil and natural gas, and particularly, to a thread joint of a surface-sealing type.
2. Description of Related Art
In general, a round thread joint and a buttress thread joint of API standard are used as a thread joint for connecting oil country tubular goods. However, since oil wells and gas wells are becoming deeper and deeper these years, thereby making excavation conditions severer, a thread joint is exposed to a large three-dimensional load and subjected to a severe stress. In a production well, in particular, where the joint is subjected to a stress of a circumferential direction from within tubes, the joint must remain air-tight under a 3-axial stress.
A round thread joint and a buttress thread joint do not sufficiently meet such demand. A surface-sealing type thread joint which includes a metal sealing surface was developed for this reason, and use of surface-sealing type thread joints has already begun in a number of wells. A structure of a surface-sealing type thread joint will now be described, referring to FIGS. 1A, 1B, 2A and 2B.
FIG. 1A is a partial broken view showing a connection part of oil country tubular goods, and FIG. 1B is an enlarged view of this portion. Tubes P and P are connected to each other by a coupling C. Each tube P includes a pin portion 10 at an end which is to be inserted into the coupling C. The coupling C includes a box portion 20 for receiving the pin portion 10. A thread joint binds the pin portion 10 and the box portion 20 by screwing.
The surface-sealing type thread joint includes a male thread 11, which tapers toward a tip with an increasingly smaller diameter, on an outer periphery. A sealing portion 12 which tapers toward a tip with an increasingly smaller diameter is formed on the tip of the male thread 11. The tip of the pin portion 10 is a shoulder portion 13 which is a partial reverse conic surface having a reverse angle with respect to a cross section which is perpendicular to an axis of the tube. On the other hand, the box portion 20 internally includes a female thread 21 which corresponds to the male thread 11. A sealing portion 22 which correspond to the sealing portion 12 is formed at a back part of the box portion 20, and at a further back part of the box portion 20, a shoulder portion 23 which corresponds to the shoulder portion 13 is formed.
When the male thread 11 is screwed into the female thread 21, the sealing portions 12 and 22 contact each other. Since this allows the shoulder portions 13 and 23 to abut each other, the pin portion 10 and the box portion 20 are sealed face to face. To generate a sufficiently large surface pressure at the sealing surfaces, a diameter difference called an interference is supplied between the sealing portion 12 of the pin side and the sealing portion 22 of the box side.
The interference S is expressed by: EQU S=D.sub.PX -D.sub.BX
D.sub.PX : the diameter of an apex of the pin side sealing portion (apex point)
P.sub.BX : the diameter of an apex of the box side sealing portion (apex point)
Therefore, S&gt;0, that is, D.sub.PX &gt;D.sub.BX. Since the interference is supplied, the sealing portions 12 and 22 start interfering each other during screwing-in (FIG. 2A). Binding completes upon abutment of the shoulder portions 13 and 23 (FIG. 2B). A stroke of an axial direction from the beginning of the interference between the sealing portions 12 and 22 until the end of the binding is referred to here as a screwing-in quantity MOS ("make-up on seal").
In such a thread joint of the metallic surface-sealing type for a tube, due to the diameter difference between the sealing portions 12 and 22, during a period from the beginning of the interference between the sealing portions 12 and 22 until the end of the binding, i.e., while screwing-in of the make-up on seal quantity MOS is executed, the sealing portions 12 and 22 slide in a spiral direction while maintaining a high surface pressure. Hence, galling is likely to be created at the sealing portions 12 and 22. When galling is generated, the sealing surfaces become rough, whereby a clearance or excessive interference is created locally and the sealing performance is deteriorated.
In a joint which is used for oil country tubular goods of a casing size which has a relatively large diameter, in particular, galling is likely to be created since a distance L.sub.S that the sealing portions 12 and 22 slide (i.e., a distance that the sealing portions 12 and 22 move with friction with each other in a spiral direction) is long and a contact surface pressure P is large. If a tube is made of stainless steel which is alloy metal of nickel, chrome and etc., or is made of pure titanium or a titanium alloy, since the heat conductivity ratio is lower than that of a steel tube and a large quantity of heat is accumulated at sliding portions, galling is often created even in a tube which is of a tubing size which is a small-size.
Japanese Patent Application Laid-Open No. 60-121385 (1985) discloses a tube joint on which an Fe plating layer and a chemical synthesis film including phosphoric acid are formed in order to prevent galling at a thread surface.
Meanwhile, Japanese Patent Application Laid-Open No. 61-286688 (1986) discloses a tube joint which satisfies the relationship: EQU L.gtoreq.(D-d) tan .gamma.
where
L: Taper Extending Size
D: Minimum Outer Diameter Of Pin
d: Maximum Inner Diameter Of Tapering Portion Of Box
.gamma.: Inclination Angle Of Tapering Portion Of Box and therefore which avoids contact at terminal surfaces. Japanese Patent Application Laid-Open No. 61-286689 (1986) discloses a tube joint which satisfies the relationship: EQU R=(m-m.sub.1).sup.2 /2(t-t.sub.1)
where
R: Radius of Curvature of Tip Portion Of Pin
m: Total Length of Tapering Outer Peripheral Surface
m.sub.1 : Effective Sealing Length Of Tapering Outer Peripheral Surface
t: Minimum Thickness Of Tapering Outer Peripheral Surface
t.sub.1 : Thickness of Ring-Shaped End Surface and therefore in which the diameter of a tip portion of a pin becomes smaller moderately when tightened by a machine and therefore galling which is created due to concentration of a tightening stress is prevented.
Japanese Patent Application Laid-Open No. 62-209291 (1987) discloses a tube thread joint which improves a galling resistance by further reducing a contact surface pressure. Japanese Patent Application Laid-Open No. 63-111388 (1988) discloses a tube thread joint in which two slope portions are formed on the pin side so that end portions do not contact each other when the end portion is inserted.
Japanese Patent Application Publication No. 6-89863 (1994) discloses a tube thread joint in which a guiding surface 42 is formed in parallel to a bottom portion of a thread between a sealing surface 48 and a thread 40 and an inclination of the sealing surface 48 is larger than this, as shown in FIG. 3, so that a damage on surfaces upon engagement of the surfaces is prevented and a dope pressure is reduced.